1. Field of Invention
The present invention relates generally to pixel based temporal video processing, and more specifically to reducing undesirable contouring artifacts due to bit limiting by applying temporal dithering at a preprocessing stage, and applying temporal filtering at a post-processing stage prior to presentation.
2. Background of Invention
Dithering is a video processing technique used to improve the quality of video images which are displayed on a device with a lower resolution lower than the original image. An example of this would be displaying 10-bit video on a 6 or 8-bit display device. Simple truncation or rounding to nearest integer produces contouring artifacts in the areas of the still video with slow spatially varying brightness or color. In order to avoid contouring artifacts, the least significant truncated bits are used to generate a dither signal. The dither signal is a single bit signal, which is added to the least significant bit of the image after truncation. By proper selection of the dithering signal, many contouring artifacts can be eliminated. Different dithering techniques exist to create dither signals. Most important techniques are evaluated with respect to the quality of the result and cost of implementation.
There are two main prior art approaches to dithering. One approach to dither generation is by use of an error diffusion algorithm. A binary threshold (equal to the half of the maximum signal amplitude) is applied to the input signal 105. The error from the threshold operation is added to the neighboring pixels so that it does not accumulate. Hence, the average dithering signal value approaches the amplitude of the input signal 105. The binary dither pattern exhibits random structure. The error can be diffused spatially, by adding four spatially neighboring weighted error terms to the input pixel prior to apply the threshold. This approach can be simplified by using two error terms. Prior art error diffusion dithering is traditionally applied spatially, such that the dithering algorithm does not change from one frame to another. In this case, although the contouring artifacts are greatly reduced, the dithering pattern is static and can be perceptually annoying.
Another known approach is to construct a binary dither signal using predetermined fixed size binary patterns. A unique dither pattern exists for each discreet input signal 105 amplitude. The average signal value in a dither pattern corresponds to the signal amplitude that this pattern represents.
Several prior art methods have been proposed to improve the visual appearance of the dither patterns by applying temporal perturbations to the dither pattern itself. One such proposal is to use pseudo-random spatial-temporal dither. A 4 by 4 by 4 dither matrix is constructed such that the mean value along the horizontal, vertical and temporal axes is always zero. This zero mean dither signal is added to input signal 105 prior to truncation. Another proposal is spatial dithering with pixel adaptive spatial dither patterns and deterministic temporal perturbations. This approach also uses predetermined dither patterns. In order to dither a two bit signal, a 2 by 2 dither matrix is proposed with the number of ones equal to the input 2-bit magnitude. The 2 by 2 matrix changes for odd and even frames. Although applying these temporal patterns reduces contouring artifacts and the visual static pattern to some extent, a greater level of reduction at a lower computational cost would be desirable.
Furthermore, dithering is typically performed at a display level, to truncate video output according to the limitations of specific display hardware. For example, where a 10 bit single is being dithered for output on an 8 bit display, no provision is made for display of that signal on hardware with a limitation greater than 8 bits. It would be further desirable to condition a signal for transmission to any output device independent of output resolution, such that the signal can be displayed at the native resolution of the device, or at a simulated higher resolution if the device supports such display.